Clean room environments in which ceiling grids containing filter panels are employed are well known in the art. Such systems generally employ dropped ceilings above which is a positive pressure plenum from which air passes through the filter panels in the ceiling grid into the clean room. Normally, in a clean room environment, it is imperative to achieve a hermetic seal for the filter panels in the ceiling grid since the particulate count in the room must normally be maintained at 10 particulates per cubic foot or less even though the air above the plenum often contains as many as 100,000 particulates, even under ideal conditions. In an attempt to provide such a hermetic seal so that no air can leak around the filter panels where they contact the supporting ceiling grid, prior art techniques have employed the mounting of filter panels in troughs filled with a vaseline like substance in an effort to provide a hermetic type seal while allowing the filter panel to be readily removed for access or replaced. Such prior art systems also employ gaskets located on the filter screen for sealing the gasket cavity in the frame mounted position of the filter panel. If such gaskets leak, however slight, or the seal is not perfect, this can totally destroy the ability of the clean room environment to maintain the desired 10 particulates per cubic foot particulate count. In an event to compensate for such potential problems, negative pressure plenums have been employed to prevent unfiltered air from entering the clean room environment should a leak occur in the filter housing, such as offered by the LS Series Type 2 laminar flow work stations commercially available from Integrated Air Systems Inc., the assignee herein. However, such a system is a suspended module and, although satisfactory from a leakage point of view, it is not readily adaptable to a suspended grid type of dropped ceiling created clean room environment. In such an environment, it is desirable not only to create a negative pressure plenum, but to do it with the least number of components, with minimal installations time, and with as modular a system as possible to readily accomodate for variations in the size of the rooms which are being converted to clean rooms. To applicant's knowledge no such satisfactory system exists in the prior art. These disadvantages of the prior art are overcome by the present invention.